What is your philosophy on life? Are you a nihilist? What about an existentialist? Do you simply not care?

Personally, I'm an existentialist. I believe that meaning is subjective and that thereby the individual determines both the worth of their life as well as its purpose. However, I also believe that the interactions between oneself and others also attach purpose to your life. Humans are social creatures and driven to interact with one another -- an act which leads to the forging of bonds. If you were to die or vanish from the face of the Earth it would impact the lives of those you possess relationships with; thus, your life has meaning to them.

It's nothing very special. Try and be nice to people, avoid eating fat, read a good book every now and then, get some walking in, and try and live together in peace and harmony with people of all creeds and nations.

Edit: No, but seriously. This is a debate that's been had for thousands upon thousands of years with no conclusive answer, and if I were to choose "other" I would mean "I haven't the foggiest." There are various veins of thought I've lent my mind to and agreed with in some part or another, and existentialism is among them. Nevertheless, I honest to goodness just could not give a completely certain answer.

my current working theory:
There is a part of you beneath your normal consciousness that you occasionally get to interact with, and that deeper self is learning and experiencing specific things in your life whether you know it or not.
Predestination isn't so much of a rigid form as it is an anticipated eventuality. The tendency is to follow your instincts and thoughts, and it would prove to be a simple matter for a higher being of some sort to tweak things to adjust the flow.
Free will is available, but most people don't rise to the level of using it.

There was a time when I not only contemplated this, but also actively tested it. The change was small but notable (compared to the prescient dream playing out at the time). The main issue I see is the difficulty in breaking from instinct, and the fact that we largely lack a broader knowledge to know if we actually succeeded or not. There were many interesting things I experienced back then related to it that tend to look like more than simple imagination.